1. Field of the Invention
This invention relates to electric elevators, and more particularly to an electric elevator car driving device comprising a combination of a sheave having non-metallic cable race linings and a controller which stops the elevator car when the cable race linings are damaged.
2. Description of the Prior Art
In electric elevator car drives, the sheave transmits power from the electric motor to the cables which carry the elevator car at one end thereof. This, it is important that the sheave have a great traction ability, which is measured in numerical terms by the ratio of the tensile forces on the tight and the slack parts of the cable on opposite sides of the sheave. In conventional electric elevator car drives, however, the sheave, which is made of metal, directly engages with the cables in grooves formed around the circumferential surface of the sheave. Thus, because the coefficient of friction between the metallic sheave and the cables is small, it is difficult to increase the traction effect of the sheave without adverse effects. For example, although an acute V-shaped groove increases the traction between the sheave and the cables, it also makes the life of the cables shorter and generates more noise during operation.
Thus, it has been proposed to fit non-metallic inserts in the sheave grooves in order to increase the coefficient of friction between the sheave grooves and the cables, and to increase the traction between the sheave and the cables. Such a sheave structure is described, for example, in R. S. Philoips, "Electric lifts", sixth edition, Pitman Publishing, in connection with FIGS. 4.8 and 4.9.
Although this measure apparently increases the traction between the sheave and the cables, it may also create a safety problem; namely the traction between the sheave and the cables is suddenly decreased when the non-metallic inserts fitted in the grooves become worn to the extent that they are broken and torn away. This sudden decrease in the traction between the sheave and the cables may cause the cables to slip, with a consequent adverse effect on the car.